The present invention relates to a horizontal automatic frequency control (AFC) circuit in a horizontal deflection circuit for driving a cathode ray tube (CRT) and particularly to a horizontal AFC circuit designed for reducing a horizontal distortion and horizontal jitter on a CRT screen.
A horizontal automatic frequency control (AFC) circuit is a primary circuit for driving a cathode ray tube (CRT) display. A conventional horizontal AFC circuit will be described referring to the relevant drawings.
FIG. 5 is a block diagram of the conventional horizontal AFC circuit. A video signal processor 301 demodulates and converts a video signal 309 received into a YUV signal or an RGB signal. A synchronous separator 302 separates a horizontal sync signal and a vertical sync signal from the video signal 309. An Hout signal circuit 304 is responsive to a horizontal (H) reference pulse 312 received from the synchronous separator 302 for generating a horizontal driving pulse 313 which drives a horizontal deflection driver 307. A video signal driver 306, upon receiving the YUV or RGB signal from the video signal processor 301, generates a video signal for driving a CRT 308. The horizontal deflection driver 307, in responsive to the horizontal driving pulse 313, controls horizontal deflection for the CRT 308. Also, the horizontal deflection driver 307 generates a flyback pulse 311. The H reference pulse 312 is synchronized with the video signal 309.
FIG. 7 is a block diagram of the Hout circuit 304. A counter 501 releases an H pulse 510 after a specific period of time from receiving the H reference pulse 312. An HSAW circuit 502 generates a ramp-up sawtooth waveform signal at an H rate from the H pulse 510. Another HSAW circuit 503 generates a ramp-up sawtooth waveform signal at an H rate from the H reference pulse 312. A comparator 504 compares the sawtooth signal 511 from the HSAW circuit 502 with a DC voltage 514 from a low pass filter (LPF) 508. When the sawtooth signal 511 is greater, the comparator 504 releases a high (H) level signal. Similarly to this, a comparator 505 compares the sawtooth signal 512 from the HSAW circuit 503 with the DC voltage 514 from the LPF 508. And when the sawtooth signal 512 is greater, the comparator 505 releases an H level signal. A reset/set (RS) flip-flop 506 is reset when the comparator 504 outputs the H level, thus turning the horizontal driving pulse 313 to a low (L) level. When the comparator 505 outputs the H level, the flip-flop 506 turns the horizontal driving pulse 313 to the H level. A multiplier 507 multiplies the flyback pulse 311 by the H reference pulse 312. An LPF 508 cuts off a high frequency component of an output 513 of the multiplier 507.
An operation of the conventional horizontal AFC circuit having the foregoing arrangement will be explained. FIG. 6 is a diagram illustrating the operation of the conventional horizontal AFC circuit. The synchronous separator 302 generates the H reference pulse 312 at horizontal rate from the video signal 309. The Hout circuit 304 retards the H reference pulse 312 for a specific time to generate the H pulse 510 and releases the horizontal driving pulse 313. The horizontal deflection driver 307, upon receiving the horizontal driving pulse 313, generates and releases the flyback pulse 311. The delay of the flyback pulse 311 from the horizontal driving pulse 313 may be varied depending on a temperature and a load. This variation causes a horizontal position change on the CRT 308.
For reducing the horizontal position change, the horizontal AFC circuit has the flyback pulse 311 synchronized in phase with the H reference pulse 312. The horizontal AFC circuit incorporates a loop circuit including a multiplier 507, an LPF 508, comparators 504 and 505, an RS flip-flop 506, and a horizontal deflection driver 307. In case that the flyback pulse 311 lags behind the H reference pulse 312, the multiplier 513 outputs a signal where a lower portion from the center in the wave amplitude is greater than an upper portion. This declines the DC voltage 514 and positions the horizontal driving pulse 313 earlier than the H reference pulse 312. As a result, a phase difference between the H reference pulse 312 and the flyback pulse 311 is eliminated, and thus the image on the CRT 308 does not move along the horizontal direction.
However, the conventional AFC circuit has the following disadvantage. When the Hout circuit 304 having an analog device is accompanied with a digital circuit in the video signal processor 301 and the synchronous separator 302, the circuit 304 may be influenced by a clock signal in the digital circuit and a change of a power source voltage. Accordingly, a jitter in the horizontal driving pulse 313 is hardly attenuated.
A horizontal automatic frequency control (AFC) circuit reducing a horizontal distortion on a cathode ray tube (CRT) and hardly be affected by a digital noise, thus declining horizontal jitter.
The AFC circuit includes the following components:
(a) A synchronous separator for generating a horizontal (H) reference pulse from a video signal;
(b) An Hout signal generator for generating, from an output of the synchronous separator, a horizontal driving pulse which drives a horizontal deflection yoke of the CRT;
(c) A horizontal deflection driver for controlling horizontal deflection of the horizontal deflection yoke in responsive to the horizontal driving pulse;
(d) A line memory into which each line of the video signal is written along a write clock (WCK) enabled, and from which the video signal is read out with a read clock (RCK) which is synchronized with the flyback pulse which determines the starting point being started from the flyback pulse; and
(e) A read clock generator for generating the RCK from the flyback pulse.